Disc files of the so-called "Winchester" type are often provided with one or more magnetic transducing heads mounted on pivoting arms so that each head sweeps an arc extending generally radially of the rotating magnetic recording disc, and traces circular "tracks" thereon.
High density recording, and the reading of it, are both easily frustrated by the presence of specks of dust or debris; therefore the apparatus is usually contained within a hermetically sealed housing, which raises a requirement for completely automatic operation of the mechanical parts of the interior apparatus, in response to electrical signals from outside, once assembly has been completed and the apparatus leaves the factory.
One complication of the hermetic sealing of the apparatus is the difficulty of preventing damage to the interior structure when the power is shut off and the apparatus is being moved or transported. When subjected to jars, shocks or vibration, the pivoting arms may pivot and whip around, damaging the extremely delicate magnetic transducing heads at the arm ends, and the thin magnetic surfaces of the discs engaged by the heads. Ordinarily one would provide a manual battening means or latch for such occasions, but the difficulties of operating such latches remotely and automatically are great.
Previously in the art, it has been customary to provide a "fail-safe" energizing means for the voice coil motor that position-drives the head arms, such as a charged condenser that is controlled to discharge, upon failure of the normal operating voltage, so as to drive the arm motor to one extreme of its operating range, where a latch may be engaged. One form for such a latch may be the well-known door-latch in which a centrally pivoting U-shaped member is toggled to open its "U" to the door-edge as the latter closes, whereupon the door strikes the far arm of the "U" and pivots it over dead-center to bring the other arm of the member into retaining position to hold the door closed.
Unfortunately, as experienced door-operators well know, if such a U-shaped member is moved accidently to closed position while the door is still open, then the door is blocked from closing and can never be latched again, without manual intervention. But such intervention would be impractical in the hermetically sealed environment of the present invention.
Another difficulty with such a latch, for use in a disk-drive environment, is that the fail-safe driving impetus is of limited strength and duration, so that the latch inventor is under constraint to keep the U-member as close to the dead-center of the toggle spring as may be possible, while the latch is open, without permitting it to be moved over-center toward closed position under the influence of external shock or vibration.